1. Field of the Invention
The present invention relates to a EFM signal compensation apparatus, and more particularly to a such apparatus for controlling the timing of an EFM signal to be recorded on an optical disk.
2. Description of Related Background Art
In forming a pit pattern by applying a laser beam to a recording medium such as an optical disk, the laser beam is controlled by an EFM signal Eight-to-Fourteen modulated by recording information.
The length of a pit formed on an optical disk by applying a laser beam changes with the diameter of the laser beam, the response characteristics of the recording medium, and the like. Therefore, a reproduced signal of a pit contains error components. To reproduce a correct signal, it is therefore necessary to finely adjust a time period during which a laser beam is applied in forming a pit. Such fine adjustment is performed using a EFM signal compensation apparatus which finely controls the timing of an EFM signal.
A conventional EFM compensation apparatus of this kind has a circuit configuration such as shown in FIG. 3.
In FIG. 3, a EFM compensation circuit 21 is constructed of, e.g., a shift register of multiple stage subordination structure having an output tap at each stage, the register being driven by input clocks. An output tap designated by a control signal from a control unit 22 is selected. An output EFM signal whose timing has been controlled by an amount corresponding to the selected output tap, is then obtained.
The conventional EFM compensation apparatus described above adjusts the delay time of the EFM signal by the amount specified by the control signal.
Read/write and other operations of an optical disk are presently defined by the standards stipulated by a so-called Orange Book which is substantially international standard formats. In some cases, such as timing adjustment of only particular EFM data, it becomes necessary to change the standards.
For example, the timings of all EFM data signals including 3T to 11T, T representing a so-called channel bit rate corresponding to the transmission rate, having been adjusted uniformly by the predetermined time period.
However, the length of a pit formed by 3T data is relatively short and its reproduced signal is low level, degrading the quality of whole reproduced signals. There arises a need of new standards including timing adjustment of only particular data (e.g., 3T data) differently from other data.
However, although the above-described conventional EFM compensation apparatus can control the timings of all EFM data signals uniformly, it cannot control the timing of a particular EFM data signal differently from other EFM data signals.